POSTERS - BLAST X - University of Utah

BLAST X Thurs. Morning Session
TWO REGULATORY PROTEINS CONTROL THE SWIM-OR-STICK SWITCH IN
ROSEOBACTERS
Robert Belas
University of Maryland Biotechnology Institute, Center of Marine Biotechnology, 701 East Pratt
St., Baltimore, MD 21202
Members of the Roseobacter clade of α-Proteobacteria are among the most abundant
and ecologically relevant marine bacteria. One of the most salient features of the roseobacters
from aspects of marine ecology is their ability to enter into close physical and physiological
relationships with “red tide” phytoplankton such as dinoflagellates. For example, Silicibacter sp.
TM1040, our model roseobacter, forms a symbiosis with the dinoflagellate Pfiesteria piscicida,
such that the dinoflagellate cannot live without TM1040. Aiding TM1040 in development of the
symbiosis is a biphasic swim-or-stick lifestyle wherein a genetic regulatory circuit controls
whether the bacteria are motile and chemotactic or sessile and develop a biofilm. Bacterial
swimming and chemotaxis behavior are initial, essential steps in establishment of the symbiosis.
Once near the host surface, motility and flagellar synthesis are downregulated, while biofilm
formation and synthesis of an antibiotic are upregulated. The abilities to swim using flagella and
to form a biofilm via adhesins have been demonstrated to be important traits for both pathogenic
and symbiotic bacteria. While it is generally agreed that motility and biofilm development are
mutually exclusive, the molecular mechanisms that underlie the lifestyle switch remain virtually
unknown for most bacterial species. We have used genetic screens to search for mutants
defective in either the motile or the sessile phenotype, and have discovered many new genes
including two previously unknown and novel regulatory proteins, FlaC and FlaD that are
envisaged to act together with cyclic dimeric GMP to play important roles in the swim-or-stick
switch. FlaC is predicted to function as a response regulator protein, with homology to a protein
of Caulobacter crescentus known to be important for cell envelope function. FlaC - cells are
skewed towards the motile phase, e.g., their populations have a greater percentage of motile
cells and fewer rosettes, and have defects in antibiotic synthesis and biofilm formation. Thus,
FlaC determines whether the switch is in the swim or stick position. FlaD is predicted to be a
MarR-type DNA-binding protein. Mutations in flaD result in nonmotile cells that synthesize but
cannot rotate their flagella, i.e., they produce paralyzed flagella. We hypothesize that FlaD is
involved in the function of the flagellar motor, either by (1) acting directly to control transcription
of the class IV fliL operon or (2) acting indirectly to control transcription or activity of a protein
that acts as a ‘clutch’ to engage or disengage the flagellar motor. The implications of FlaC and
FlaD activities in the swim-or-stick strategy and their impact on the symbiosis will be discussed.
37